ABSTRACT VRC01-class broadly neutralizing antibodies (bNAbs) recognize a conserved epitope within the CD4- binding site of HIV-1 Env. They are among the most potent bNAbs known and protect animals from experimental S/HIV infection, making them a highly attractive type of antibody to elicit by vaccination. They have been isolated from multiple HIV-1-infected subjects, but are all derived from the same VH1-2 allele (*02) and a small number of light chains, all of which express a 5 amino-acid CDRL3. In contrast to the mature, fully mutated forms of VRC01-class antibodies, their inferred germline forms do not recognize Env and do not neutralize HIV-1. This led to the hypothesis that previous recombinant Env immunogens were ineffective in activating nave B cells expressing germline VRC01-class B cell receptors (BCRs), which may, in part, explain why such immunogens have not elicited VRC01-like antibody responses in vaccine studies. We reported on the design of a clade C-derived Env protein (426c Core) that binds germline VRC01-class antibodies and initiates the expansion of nave B cells expressing the corresponding BCRs in vivo, but is insufficient to induce the maturation of these BCRs towards their neutralizing forms. A major hurdle to overcome in order to elicit any bNAbs through immunization is due to steric restrictions imposed by glycans present at the conserved glycosylation site N276 in Loop D of Env. These glycans limit access to the epitope recognized by germline VRC01-class antibodies, but as the antibodies undergo somatic hypermutation and affinity-based selection they ?learn? how to bypass this steric block. The success of our immunization strategies to elicit VRC01-class bNAbs will therefore depend on our ability to guide the evolution of germline VRC01-class antibodies along particular maturation pathways to bypass the N276 glycan-imposed restrictions. In this Project we propose to use concepts and reagents, not tested previously, in an effort to overcome these major obstacles preventing the generation of VRC01-class antibodies by immunization. Specifically, we propose to use anti-idiotypic monoclonal antibodies (aiMAbs) against the germline VRC01-class antibodies to specifically increase the frequency of VRC01-expressing B cells prior to immunization with the germline-binding? 426c Core immunogen, followed by booster immunizations with Env-based reagents that select for VRC01-class B cells that can bypass the restrictions imposed by the N276 glycans. Our studies will be performed in an iterative fashion in diverse animal models that express VRC01-class BCRs, including mice engineered to express a polyclonal human BCR repertoire.